1. Field of Invention
The invention relates to a conversion mixer, and, in particular, to a conversion mixer for converting a high-frequency input signal into a low-frequency output signal.
2. Related Art
In microwave communication technology, because radio frequency (RF) signals in a transceiver cannot be easily processed, the received radio frequency signal is modulated and down-sampled to a baseband signal to facilitate the subsequent signal processing procedures.
Currently, a conversion mixer for mixing and modulating the radio frequency signal with a local oscillation signal and thus generating a new middle signal is often added to an RF front-end circuit of the transceiver. In addition, the conversion mixer also controls important parameters, such as noise figure isolation, 1 dB compression point and third-order alternating modulation point (IP3) of a transceiver circuit. So, the conversion mixer plays an important role in the transceiver. Currently, the conversion mixer is widely used in the radio frequency circuit in a chip of a mobile phone.
With regard to the conventional conversion mixers, active conversion mixers are the most common conversion mixers, and double balanced mixers, also referred to as Gilbert-cell mixers, are the most common of the active conversion mixers.
Referring to FIG. 1, a Gilbert-cell mixer 1 includes a BJT differential pair circuit 11, a current switching circuit 12 and a loading circuit 13. The BJT differential pair circuit 11 and the current switching circuit 12 are respectively composed of transistors Q5 and Q6, and transistors Q1 to Q4. The BJT differential pair circuit 11 receives a couple of externally inputted high-frequency radio frequency signals RF+ and RF−, and the current switching circuit 12 has two inversely switching sub-current switching circuits 121 for respectively receiving a couple of local oscillation signals LO+ and LO−.
Herein, the Gilbert-cell mixer 1 operates as follows. The couple of radio frequency signals RF+ and RF− is received by the BJT differential pair circuit 11, and a couple of radio frequency current signals I.sub.R+ and I.sub.R− is generated and outputted. The current switching circuit 12 receives the couple of radio frequency current signals I.sub.R+ and I.sub.R− and the couple of local oscillation signals LO+ and LO−, and mixes the couple of radio frequency current signals I.sub.R+and I.sub.R−with the couple of local oscillation signals LO+ and LO− using the inversely switching sub-current switching circuits 121 in the current switching circuit 12. Then, a cross-coupling procedure is performed so that a couple of baseband current signals IF+ and IF− is outputted from the loading circuit 13.
Because a larger current has to be provided to drive the Gilbert-cell mixer 1, when the provided current I flows through the loading circuit 13, the voltage V′ (V′=VDD−V) provided by a supply power VDD to the BJT differential pair circuit 11 and the current switching circuit 12 is equal to the supply power VDD minus the voltage V (V=I×RL) of the loading circuit 13. If the residual voltage V′ of the supply power VDD is too low, the operation regions of the transistors of the BJT differential pair circuit 11 and the current switching circuit 12 may be influenced so that the couple of baseband current signals IF+ and IF− is influenced by the loading circuit 13 more greatly than by the couple of radio frequency signals RF+ and RF−, thereby causing a dead zone phenomenon.
In addition, the couple of baseband current signals IF+ and IF− is restricted by the size of the range of the voltage value of the loading circuit 13 so that the operation range of the couple of baseband current signals IF+ and IF− is small, and the couple of baseband current signals IF+ and IF− have non-linear outputs. Also, because the Gilbert-cell mixer 1 needs to utilize the higher supply current I, its power consumption becomes a serious problem.
Thus, it is an important subject to provide a conversion mixer capable of reducing the power consumption and having linear output signals with a larger operational range.